Industrial gas turbine engines include a compressor for compressing air, a combustor arrangement for combusting a mixture of fuel and the compressed air, and a turbine for extracting energy from the combustion gases. The turbine section includes rows of blades secured to a rotor shaft, all of which are turned by the combustion gases in the energy extraction process. Between the rows of turbine blades are rows of stationary vanes that properly orient the combustion gases as the combustion gases travel within the turbine. Each row of vanes includes vane segments, each of which has an inner shroud and an outer shroud secured to opposite ends of at least one airfoil. In many turbines the airfoils may be cooled via an internal cooling channel and backsides of the shrouds may also be cooled. These cooled airfoils may be part of the first and even second rows of turbine blades. Cooling may include convective cooling via a flow of cooling air over the surface to be cooled, and/or impingement cooling via an impingement plate inserts placed inside the airfoil's internal cooling channel and adjacent the backside of the shroud.
The airfoil and shrouds may be cast together thereby forming a monolithic vane segment. Alternately, the airfoil and shrouds may be case separately and then welded together. The airfoil's internal channel requires the use of a ceramic core to create the channel and define the internal surface configuration during the casting process. The remainder of the vane segment surfaces, including those of the inner and outer shrouds, is typically defined by a ceramic shell that is formed around a wax pattern of the vane segment, where the wax pattern is formed around the ceramic core. The wax pattern is then removed, leaving a void in the shape of the vane segment, where the ceramic core defines surface of the airfoil's interior channel and the ceramic shell defines the remainder of the surface of the vane segment.
Small features are desirable on some of the surfaces of the vane segment, including the airfoil's internal channel and the backside surface of the shroud, because they can be used in conjunction with impingement jets to improve the impingement cooling. The small features can be readily formed in the surface of the airfoil's internal channel by the ceramic core because the small features on the ceramic core survive the steps leading to the final casting, and are impressed directly onto the final vane segment. The small features on the backside surface of the shroud, however, would be formed by the wax pattern, since the wax pattern defines the backside surface of the shroud. The wax pattern is a soft material. For this reason if the small features are impressed on the surface of the wax pattern and then subject to the dipping process, during which the ceramic shell is formed, the small features are distorted and/or lost. Since small features in wax patterns cannot survive the dipping process, the surfaces of the vane segment that are defined by the shell, (which are, in turn, defined by the wax pattern), cannot have small features when the small features would result from a wax pattern that is exposed to the dipping process.
One technique that has been used to overcome this problem is to use a separately-cast, discrete ceramic insert to form the small features on the shroud backside surfaces. The wax pattern is formed around the ceramic core and the ceramic insert and the shell removed, leaving a void for the vane segment. In this method the ceramic core define the airfoil's internal surface and its small features, and the ceramic insert defines the shroud backside surface and its small features, and the remainder of the vane segment surface is formed by the shell. However, the position of the ceramic insert is difficult to control precisely and the quality of the casting is less than acceptable when a ceramic insert is used. For the foregoing reasons there is room in the art for improvement.